1. Field of the Invention
The present invention relates to a mold for extrusion forming of ceramic articles which is preferably used for extrusion forming of, for example, ceramic honeycomb structures. More particularly, it relates to a mold for extrusion forming of ceramic articles which is excellent in wear resistance and can remarkably decrease forming defects in the vicinity of outer peripheries thereof which are easily generated during the extrusion forming of the ceramic articles having highly dense honeycomb-shaped thin walls.
2. Description of the Related Art
A ceramic honeycomb structure is mainly used as a catalyst carrier which is used to remove atmospheric contamination substances in an exhaust gas discharged from an internal combustion engine, a boiler or the like, or as a filter for collecting diesel fine particles.
Heretofore, as a mold for use in extrusion forming of such a ceramic honeycomb structure, there has been known a mold for forming a honeycomb article which includes a die having groove-like slits formed by cell blocks on the surface of a mother material of a stainless steel, SKD or the like, and including back holes provided on the back surface of the die so as to communicate with the slits.
In such a mold for forming the honeycomb article, a surface membrane is used to adjust the slit width of each cell block and to improve the durability of the die. For example, the surface membrane can be obtained by forming a nickel plating layer on the surface of a cell block main body, and then forming, on the nickel plating film, a CVD or PVD layer containing as main components one or more substances selected from the group consisting of TiC, TiN and TiCN, or a composite plating layer where hard powder of SiC, diamond, CBN or the like is dispersed in a nickel plating film (e.g., see Japanese Patent Application Laid-Open No. 7-52129).
In Japanese Patent Application Laid-Open No. 7-52129, a mold for extrusion forming of the ceramic honeycomb structure shown in FIG. 11 is disclosed. The extruding direction of a kneaded ceramic clay is a direction from the upside to the downside, and a back pressing portion 3 is capable of adjusting the amount of the kneaded clay to be fed into the mold. A die 1 is fixed by a pressing portion 2, and the kneaded clay extruded through a gap 5 between the die 1 and the pressing portion 2 adjustably forms the outer peripheral portion of a formed ceramic article 4 through an inclined face 6 and a facing face 7.
Moreover, in Japanese Patent Application Laid-Open No. 2002-283327, a vertically sectional view of a honeycomb structure forming device 50 shown in FIG. 2 is described. In FIG. 2, the honeycomb structure forming device 50 includes a die 54 having back holes 53 for supplying a forming material and slits 52 for extruding the forming material, and a pressing plate 55 provided on the downstream side of the die 54. The material is continuously extruded using this device to manufacture a honeycomb structure 61.
In the honeycomb structure forming device 50, the die 54 is constituted of an inner portion 71 and an outer peripheral portion 72, the inner portion 71 protrudes toward the downstream side (to the downside in FIG. 2) to form a stepped portion 75 between the inner portion and the outer peripheral portion 72, the inner portion 71 is provided with slits 73 for forming a honeycomb structure, and the outer peripheral portion 72 is provided with slits 74 shorter than the slits 73. Moreover, a gap portion 57 for forming the outer wall of the honeycomb structure is formed between the die 54 and the pressing plate 55. It is to be noted that a pressing jig 58 and a back pressing plate 59 constitute a holder for setting the die 54 and the pressing plate 55.
During the extrusion by this honeycomb structure forming device 50, the forming material is extruded from the upstream side (the upside in FIG. 2) of the die 54 to the downstream side through the die 54 by an extruder (not shown). Moreover, the forming material extruded from the slits 73 disposed in the inner portion 71 of the die 54 opened on the downstream side forms the honeycomb structure constituted of a large number of cells. On the other hand, the honeycomb shape of the forming material extruded from the slits 74 disposed in the outer peripheral portion 72 of the die 54 is collapsed by the function of the gap portion 57. Moreover, when a proceeding direction is changed from the extruding direction to a stepped portion 75 direction and the pressing plate 55 is opened, the proceeding direction is changed to the extruding direction again to form an outer wall which surrounds the cells.
Moreover, in the manufacturing method of the die for forming the ceramic article by the extrusion disclosed in Japanese Patent No. 3648031, to precisely form the back holes which are provided in the die and through which the kneaded clay is supplied, electrolytic etching (electrical chemical machining: ECM processing) is used. Moreover, a method is disclosed in which slits are formed in the surface of the die for forming the ceramic article by the extrusion, and the surface of the die is subjected to electrolytic plating and then electroless plating.
Furthermore, in recent years, to improve the capability of removing the atmospheric contamination substances in accordance with a stricter exhaust gas environmental standard, there has been a demand for the higher density and precision of the thin walls of the honeycomb structure for use in a filter for purifying the exhaust gas.
FIG. 3 is a schematic plan view of the die 1 for use in a mold for forming a honeycomb structure as viewed from a supply end side provided with back holes 9 through which the kneaded clay is supplied. Heretofore, any problem has not been raised, but it has been found that in a case where the back holes 9 are formed by the ECM processing in order to form the die 1 in which a distance between the centers of the back holes is narrowed (from 0.5 to 1.8 mm) as shown in FIG. 3 to satisfy the demand for the higher density and precision of the thin walls of the honeycomb structure, unevenness of about 100 μm to 200 μm is generated around the back holes 9 in the supply end surface. FIG. 4 shows an enlarged region A of FIG. 3, and is a schematic plan view showing the unevenness around the back holes 9 in the supply end of the die 1 for use in the mold for forming the honeycomb structure. Regions α in FIG. 4 have a relatively high protruding state as compared with a surrounding area, and regions β have a lowly recessed state as compared with the regions α. These unevenness is generated owing to the EMC processing, but there is not any problem in a case where there is a large space between the back holes. As shown in FIG. 4, the regions β where there is a small space between the back holes 9 have the recessed state, and the regions α where there is a large space between the back holes 9 have the protruding state.
In a case where the honeycomb structure is formed using the mold for forming the ceramic article, the mold including the die 1, as shown in FIG. 10, a forming defect is easily generated in the vicinity of an outer wall 62 of a honeycomb structure 61. Specifically, in addition to a problem that a streak-like forming defect is generated in the surface of the outer wall, the width of a partition wall 67 close to the outer wall decreases, or easily fluctuates. When the first to fourth cells from the outer wall 62 of the honeycomb structure are cells 63, 64, 65 and 66 in the drawing, approximately the first to fifth cells easily cause the forming defects over the whole periphery of the outer wall, and the width of the partition wall 67 closer to the outer wall tends to decrease. Consequently, the decrease of the isostatic strength of the honeycomb structure 61 is caused.
This phenomenon will be described with reference to FIGS. 1 and 5 to 7 showing the enlarged vicinity of the outer peripheral portion 72 of the mold for forming the ceramic article, the mold having a constitution similar to that of the conventional honeycomb structure forming device 50 shown in the vertically sectional view of FIG. 2. In the mold for forming the honeycomb structure partially shown in FIG. 1, the back pressing portion 3 is constituted of a back pressing plate 12 and a back spacer 13, and the pressing portion 2 is constituted of a pressing plate 11 and a spacer 10, so that the die 1 is fixed. In the die 1, a kneaded clay supply end 22 is provided with a plurality of back holes 9, and an extrusion end 21 is provided with slits 8 which communicate with these back holes 9 to form the formed ceramic article by the extrusion.
FIG. 5 shows the enlarged vicinity of the outer periphery of the supply end 22 of a mold for forming a ceramic article partially shown in FIG. 1, and is a schematically enlarged sectional view cut along a straight line H of FIG. 3 in a vertical plane. The straight line H extends through the regions a raised in the protruding state in the uneven die surface, and the surface of the supply end 22 of the die 1 overlaps with the back spacer 13 without any gap therebetween, so that the kneaded clay does not flow between the adjacent back holes 9 of the corresponding portion. The flow of the kneaded clay is blocked by the back pressing portion 3 in the vicinity of the outer periphery (on the left side in the drawing), and the kneaded clay having a flow rate adjusted flows from the upside to the downside in an inner peripheral portion as shown by arrows.
FIG. 6 shows the enlarged vicinity of the outer periphery of the supply end 22 of the mold for forming the ceramic article partially shown in FIG. 1, and is a schematically enlarged sectional view cut along a straight line L of FIG. 3 in a vertical plane. The straight line L extends through the regions β lowered in the recessed state in the uneven die surface, and a gap is generated in a portion where the surface of the supply end 22 of the die 1 overlaps with the back spacer 13, so that the kneaded clay flows between the adjacent back holes 9 of this portion. The gap is generated between the supply end 22 and the back spacer 13 as shown in FIG. 6 because the regions β are lower than the regions α as shown in FIG. 3. In consequence, the flow of the kneaded clay is not sufficiently adjusted by the back pressing portion 3, and the kneaded clay is discharged to the back holes 9 in the vicinity of the outer periphery (on the left side in the drawing) in a lateral direction as shown by arrows in the drawing.
Thus, since the unevenness is generated in the surface of the supply end 22 of the die 1 as shown by the region A of FIG. 4, a gap is generated between the back pressing portion 3 and the recessed portion of the supply end 22 in a case where the back pressing portion 3 for adjusting the amount of the kneaded clay to be supplied covers the surface of the supply end 22 via the back spacer 13. Furthermore, the clay flows from this gap in the lateral direction as shown in FIG. 6, which results in a problem that the back spacer 13 is worn owing to an operation over a long period as shown in FIG. 7. When the amount of the kneaded clay to be supplied is not sufficiently adjusted by the back pressing portion 3, the kneaded clay is non-uniformly supplied to the outer wall portion of the formed honeycomb article, and the kneaded clay is not sufficiently supplied to the partition walls of the cells in the vicinity of the outer wall. Therefore, in particular, the thicknesses of the partition walls of the first to fifth cells from the outer wall decrease, and the decrease of the isostatic strength is incurred in a case where the honeycomb structure is prepared using the ceramic article formed by the extrusion.